You can drive a circle to get to the location and get back to where you started. Go to North Berwick and take Route 4 north toward Sanford. As you head out of town, at about three miles or so look for Quarry Road (or, as Mainers put it, The Quarry Road) on your right. Take Quarry Road and continue on to a crossroad. Turn right, and you'll be on Perry Oliver Road (The Perry Oliver Road). About a mile in, the road crosses the B&M right-of-way via a filled-in overpass. You can't miss the tell-tale, especially at this time of year, as it would stand out rather starkly against the snow. To get back to where you began, stay on Perry Oliver Road to Bragdon Road, turn right, then right again at the fork, and you'll wind up on Route 9 heading back into North Berwick. (You'll cross the current B&M/PanAm main line twice.)
You can trace many miles of the old Eastern, all the way from Eliot to South Portland, by car with a DeLorme Maine Atlas at your side. The entire roadbed (except for the far southern end, which is now Route 236) is delineated. Much of it is now the Eastern Bike Trail.
Happy hunting!